The invention relates to strippable systems comprising at least one pressure sensitive adhesive based on at least one block copolymer.
For the residueless and nondestructive redetachment of self-adhesive tapes by stretching parallel to their bond plane, very particular requirements are imposed on such self-adhesive tapes (also referred to below as strippable self-adhesive tapes). First, such adhesive tapes must be highly extensible, since it is only the stretching deformation which allows the desired detachment from the substrate. Further, such self-adhesive tapes need high cohesion, so that they do not tear under the tensile stress which is necessary in the detachment process. Of special significance is the requirement that the stretching of the self-adhesive tapes on detachment is accompanied by a significant loss of tack. Only such a loss of tack ensures that the stretched self-adhesive strip during the detachment process does not stick in the stretched state to one of the bond partners in the extended state. Absent such a loss of tack, self-adhesive tapes bonded in this way could in many cases no longer be nondestructively removable from the joint.
Self-adhesive tapes redetachable residuelessly and nondestructively by extensive stretching in their bond plane from a wide variety of substrates find broad application in the reversible adhesive sticking of articles which are light to moderately heavy, particularly in the home and office sectors. The strippable self-adhesive tape products described in the patent literature make use of single-layer and multilayer constructions. Single-layer products are described, for example, in DE 33 31 016 A1. The multilayer product constructions can be subdivided into those for which the adhesive strip tensile strength that is required for the residueless and nondestructive detachment process is mediated essentially by a backing sheet or a foam material backingxe2x80x94examples of aforementioned self-adhesive tapes can be found, inter alia, in U.S. Pat. No. 4,024,312, WO 92/11332, WO 92/11333, WO 95/06691, and DE 195 31 696 A1xe2x80x94and those for which the tensile strength is mediated substantially by way of the pressure sensitive adhesive. Examples of the latter products are described, inter alia, in DE 196 49 727 A1, DE 196 49 728 A1, DE 196 49 729 A1, DE 197 20 145 A1, and DE 198 20 858 A1. DE 197 08 366 A1 describes multilayer strippable self-adhesive tapes which are composed of a plurality of layers and comprise styrene block copolymers as base elastomer. Reference below to styrene block copolymers shall be taken to mean the class of materials comprising block copolymers composed of at least one polystyrene block and at least one polydiene block (or a partially or fully hydrogenated analog thereof). As compared with DE 33 31 016 A1, such multilayer construction makes it possible in particular to exert selective control over selected parameters which are significant properties for strippable self-adhesive tapes, such as the stripping force, the adhesion, and also the aging stability with respect, for example, to ozonolysis.
Specific embodiments and applications of aforementioned strippable self-adhesive tapes are described in DE42 22 849A1, DE44 28 587A1, DE44 31 914A1, DE 198 42 864 A1, DE 198 42 865 A1, DE 199 38 693 A1, and WO 98/03601 A1, and also in DE 42 33 872 A1, DE 195 11 288 A1, U.S. Pat. No. 5,672,402 A, and WO 94/21157 A1.
While the patent literature presented above describes a large number of different pressure sensitive adhesive systems examples including those based on natural rubber/resin blends, on styrene block copolymer/resin blends, and on random acrylic copolymers, it is nevertheless the case that commercially available strippable self-adhesive tapes use exclusively pressure sensitive adhesives based on styrene block copolymer/resin blends as, for instance, do the products Command(copyright) Adhesive from Minnesota Mining and Manufacturing Co. Inc., St. Paul (USA), the tesa(copyright) Powerstrips(copyright) from Beiersdorf AG, Hamburg (D), and Plastofix(copyright) Formula Force 1000 from Plasto S. A. (F). Apart from the tesa(copyright) Powerstrips(copyright) Deco, which comprise polymers based on saturated elastomer blocks, these systems use styrene block copolymers based on polymer blocks which are unsaturated in the elastomer block.
Advantages of such styrene block copolymer based pressure sensitive adhesives for use in strippable self-adhesive tapes are:
their very high extensibility. Elongations at break are generally greater than 400%, preferably greater than 600%, with particular preference greater than 800%.
the simultaneously very low stress values at moderate degrees of stretching in the range between about 200% and 400% elongation, which, among other things, allow very low detachment forces (stripping forces).
their tensile strength, which despite high extensibility and low stress values at abovementioned average degrees of stretching is very high and which constitutes an essential prerequisite for the residueless and nondestructive redetachment process, especially if the pressure sensitive adhesive itself contributes to the tensile strength of the self-adhesive tape.
their ability to exhibit when being stretched a significantly reduced tack, which is an essential prerequisite for an absence of sticking to one of the substrates while in the stretched state during removal from the joint. Self-adhesive strips which do stick in this way on extension can in many cases no longer be removed nondestructively from a joint by further stretching.
the outstanding bond strengths they can achieve on a variety of substrates having a very wide range of polarities.
Advantages of single-layer strippable self-adhesive tapes based on styrene block copolymers are the simple construction of the product, which is obtainable by a very simple, cost-effective production process; the realization of a wide thickness range; the possibility in principle of producing transparent adhesive tapes, especially tapes with water-clear transparency; and, for the production of punched products, a simple and readily manageable punching process. Advantages of multilayer self-adhesive tapes, consisting of a plurality of styrene block copolymer based layers, are their easy obtainability by coextrusion and the wide variability of layer thicknesses and layer compositions, which allows substantially independent control of surface properties (adhesion) and volume properties (cohesion).
Disadvantages of styrene block copolymer based pressure sensitive adhesives, when using systems which utilize unsaturated polymer chains in the elastomer block, e.g., when using styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) block copolymers, include the poor aging stability with respect to ultraviolet radiation and also to thermooxidative degradation and to ozonolysis. Accordingly, it is completely impossible to utilize the advantage of self-adhesive tapes possessing water-clear transparency, because the pressure sensitive adhesive has to be protected from the incidence of light. This is done, for example, by means of light-absorbing additives such as titanium dioxide, whose compounding leads to nontransparent products. It is true that the aging stability of styrene block copolymers containing chemically saturated elastomer blocks, examples being the hydrogenated analogs of SBS and SIS, styrene-ethylene/butylene-styrene (SEBS) and styrene-ethylene/propylene-styrene (SEPS), is much improved, they therefore typically need no light-absorbing additives, and so can be processed more easily to water-clear products. A disadvantage found, however, is typically that the good balance between bond strength and low redetachment forces, which SIS and SBS based pressure sensitive adhesives are known to exhibit, is not achieved. At the same time, in the absence of plasticizers (e.g., liquid resins, aliphatic oils), which are desirable for many formulations, the selection of sufficiently compatible tackifier resins in comparison, say, to SIS is markedly restricted.
A further disadvantage of strippable self-adhesive tapes which utilize styrene block copolymer based pressure sensitive adhesives is that, if the pressure sensitive adhesives are exposed to UV prior to their actual use, they can suffer a marked reduction in achievable bond strengths and, accordingly, a not inconsiderable expense in respect of a UV-impermeable packaging is incurred if the customer is to be guaranteed a uniformly high bonding performance. The above comments apply not only to using styrene block copolymers with elastomer block unsaturation, but also, for example, when using SEBS and SEPS. When bonding to UV-transparent substrates, such as window glass or transparent plastics surfaces, and especially when using styrene block copolymers with elastomer block unsaturation, there exists the additional risk following UV exposure that the detachment forces will rise to such an extent that the adhesive strips cannot be redetached residuelessly and nondestructively, and the adhesive strip tears during the redetachment process.
A further disadvantage of strippable self-adhesive tapes with pressure sensitive adhesives based on styrene block copolymers is the need to blend the styrene block copolymers with components which have a low molecular mass, and are therefore capable of migration, in order to set the desired pressure sensitive adhesion properties. Especially when bonding to sensitive substrates, migration of components in the tackifier resins or migration of plasticizer can result in irreversible changes to the bond substrates: for example, to greasing or instances of discoloration on a wallpaper substrate coated with white emulsion paint. Instances of discoloration may also occur, inter alia, as a result of the use of antioxidants or UV protectants, which are typically used to stabilize such pressure sensitive adhesives and/or even the raw materials of such adhesives.
Partly as a consequence of their saturated polymer backbone, acrylic copolymers possess an excellent aging stability, with respect both to ultraviolet radiation and to thermooxidative aging. At the same time, the pressure sensitive adhesion properties and also specific property features can be controlled within a very wide range by way, for example, of the comonomer composition, the nature and extent of crosslinking and, too, the optional addition of tackifier resins. Their successful use in strippable self-adhesive tapes is therefore particularly attractive.
The use of polyacrylate pressure sensitive adhesives for strippable self-adhesive tapes is described, inter alia, in WO 92/11332 A1, WO 92/11333 A1, WO 95/06691 A1, and DE 195 31 696 A1. The pressure sensitive adhesives set out therein, however, consist of random acrylic copolymers which do not per se offer sufficiently high cohesion to satisfy the requirements. Consequently, adhesive strips based on these adhesives always comprise highly extensible interposed carriers of high tensile strength.
WO 92/11332 A1 describes strippable self-adhesive tapes with an interposed film carrier which use photopolymerized acrylic pressure sensitive adhesives. Polyacrylate pressure sensitive adhesives of this kind and their preparation processes, however, give such products specific disadvantages which have very disruptive consequences in practice. The heat of reaction given off during UV polymerization of an expanse of acrylates can damage the carrier or cause it to undergo corrugation. This applies particularly to double-sided adhesive tapes. The possibility of compounding with, for example, tackifier resins is greatly restricted, since these resins disrupt the UV polymerization. Another disadvantage is the unavoidable crosslinking profile of the adhesive: normally, UV polymerization is accomplished by UV irradiation from the adhesive side. As a result, adhesive films with a higher level of crosslinking at the adhesive surface than toward the carrier are obtained. The consequences are a reduced tack and poor anchoring of the adhesive. If exposure to ultraviolet light is to take place through the carrier, which results in improved tack, adhesion, and adhesive anchoring, the carrier must be UV permeable. Many carriers, however, are not particularly UV permeable or are damaged by UV light. Overall, therefore, the product properties are such that prolonged bonding in consistent quality is not ensured: this is demonstrated in WO 92/11332 A1 on page 19, table 2, column 6, by the residues of adhesive which remain at the edges of the bond (footnote a), if the products do not tear anyway (footnote b).
WO 92/11333 A1, WO 95/06691 A1, and DE 195 31 696 A1 describe strippable self-adhesive tapes which utilize a broad combination of conventional acrylic copolymers possessing pressure sensitive adhesion. Described inter alia are radiation-chemically crosslinkable acrylic copolymers or those which, for example, utilize heat-initiated crosslinking mechanisms. A disadvantage of all of the examples set out is the need for chemical crosslinking or radiation-chemical crosslinking in order to generate sufficient cohesion in the pressure sensitive adhesives. Despite crosslinking, however, acrylic pressure sensitive adhesives of this kind exhibit tensile stress/elongation characteristics which are very disadvantageous for a strippable self-adhesive tape, and, consequently, low tensile strengths, with corresponding disadvantages for a tearfree detachment process, necessitating the use of a backing layer. A further disadvantage is a marked permanent deformation of the abovementioned pressure sensitive adhesives at high stretch, i.e., flow of the pressure sensitive adhesive film at high deformations. Such high deformations automatically occur, however, during the process of detachment by extensive stretching. The aforementioned tensile stress/elongation characteristics of such pressure sensitive adhesives may be the essential reason for the low loss of tack on stretching which is observed in acrylic adhesives and also for the high detachment forces (stripping forces) in comparison to styrene block copolymer pressure sensitive adhesives, since a correspondingly greater amount of the energy consumed in the detachment process is dissipated irreversibly in the adhesive.
Acrylic polymers obtained by UV-induced polymerization (see, for example, WO 92/11332 A1) or crosslinked by means of UV radiation (see, for example, UV-acResin(copyright), BASF(copyright)), typically cannot be processed to layers with a thickness of more than 100 xcexcm, since above such thicknesses the intensity of the penetrating UV radiation is no longer sufficient for crosslinking.
A generally critical disadvantage of the use of acrylic copolymers as strippable self-adhesive tapes are their specific tensile stress/elongation characteristics, typically characterized by low tensile strengths. Particularly under high stretch, they commonly exhibit a much lower resilience than styrene block copolymer based pressure sensitive adhesives. At the same time, stretching of acrylic copolymer based pressure sensitive adhesives of this kind is observed to be accompanied by a much lower loss of tack than in the case of styrene block copolymer based pressure sensitive adhesives (see Examples).
Such a lower loss of tack, however, has been found from experience to cause significant impairment to the detachment properties. This applies in particular to double-sided pressure-sensitively self-adhesive tapes: it is virtually impossible during practical detachment to avoid contact between the adhesive strip to be removed from the joint and one of the substrate surfaces. If, however, such double-sided self-adhesive strips stick to one of the substrates in the extended state, as a result of their high tack even in the extended state during removal from the joint, then it is in many cases no longer possible to remove them from the joint; instead, they tear during the attempt to stretch them further. Likewise, self-adhesive strips with only a low loss of tack on stretching exhibit high frictional forces on contact with their substrate surfaces, which can lead to entry cracks in the adhesive surface and, subsequently, to tearing of the self-adhesive strips.
Because of their high aging stability and the option to manage without migratable adhesive components of low molecular mass, strippable self-adhesive tapes featuring pressure sensitive adhesives based on acrylic copolymers are of particular interest for use in reusable strippable self-adhesive strips. DE 195 31 696 A1 describes strippable adhesive strips comprising an elastic, styrene block copolymer core to either side of which pressure-sensitively adhesive, acrylate-containing outer layers have been laminated. However, it is found for such systems that the loss of tack on stretching is too low to give strippable self-adhesive strips which can be detached from the substrate residuelessly and nondestructively. Whether the acrylate-containing outer layer is uncrosslinked or crosslinked chemically is immaterial, since in both cases the tensile stress/elongation characteristics of the outer layer differ unfavorably from those of the middle layer.
Aging stability and high water resistance, in contrast, are the classic advantages of pressure sensitive adhesives based on acrylate. Pressure sensitive adhesive tapes of high bond strength can be produced by targeted copolymerization with different acrylates. Absent crosslinking, however, the cohesion of such adhesives is too low. Accordingly, polyacrylates are crosslinked chemically in order to ensure a certain minimum cohesion (WO 92/11332 A1).
Through the changeover to hotmelt processes, which are used to produce pressure sensitive adhesives on account, inter alia, of the fact that they permit realization of high film thicknesses, some of the crosslinking mechanisms employed to date for polyacrylate pressure sensitive adhesives become superfluous. For example, thermal crosslinking techniques with metal chelates or polyfunctional isocyanates, which are very widespread with the solvent systems, can no longer be employed. At present, therefore, the crosslinking of polyacrylate pressure sensitive adhesives by exposure to ultraviolet light (UV) or to electron beams (EB) is being forced forward, the latter being known as electron beam curing (EBC). These two crosslinking methods, however, are not without their disadvantages. For instance, electron beams cause partial or complete destruction to the backings of the pressure sensitive adhesive tapes, and UV crosslinking is possible only up to a film thickness of 100 xcexcm, particularly with resin-blended systems. Moreover, EBC is efficient, with no loss in tack, only for polymers of high average molar mass.
Acrylate-containing block copolymers elegantly combine the advantageous properties of conventional acrylic copolymers (aging stability, water-clear transparency, inherent pressure-sensitive adhesion) and styrene block copolymers (thermoreversible physical crosslinking, high cohesion). Various synthetic approaches have been described for realizing such systems. U.S. Pat. No. 5,314,962 A describes a photoinitiated INIFERTER process (an additive-controlled radical polymerization method; cf. also T. Otsu, Journal of Polymer Science A Polymer Chemistry, 2000, 38, 2121); U.S. Pat. No. 5,264,527 A and JP 11302617 A1 describe a preparation by anionic polymerization; EP 824 110 A1 and U.S. Pat. No. 6,069,205 A describe processes based on ATRP (atom transfer radical polymerization); EP 921 170 A1 describes a radical polymerization process controlled by stable nitroxide radicals; WO 98/01478 A1 describes a further polymerization method operating by a controlled radical mechanism (RAFT, reversible addition-fragmentation chain transfer); these processes are mentioned here by way of example (K. Matyjaszewski (ed.), Controlled/Living Radical Polymerization, ACS Symposium Series 768, 2000, ACS, Washington D.C.). Some of the studies cited propose the use of acrylic block copolymers as elastomers for pressure sensitive adhesives. The cohesion needed for pressure sensitive adhesives is achieved in the acrylic block copolymers by domain formation in the endblocks, which have a softening temperature situated well above room temperature. Since, however, this domain formation is thermoreversible, such materials can be processed as hotmelts (EP 921 170 A1, WO 00/39233 A1).
In WO 00/39233 A1, block copolymers are prepared and processed as hotmelt pressure sensitive adhesives, and self-adhesive tape products are produced from them. The invention embraces block copolymers composed of at least two A blocks and one B block, the block copolymer being blended with at least 40% resin. As a result of this high resin fraction, however, the glass transition temperature of the polymer is sharply raised, so that these materials are very hard and, consequently, have low bond strengths and require high deformation forces for stretching, for example, and so are unsuited to use as strippable products.